Ceramic heater

ABSTRACT

A ceramic heater has a heater element made of a mixture including MoSi 2  and Si 3  N 4  powder as a main ingredient and silica (SiO 2 ) powder as an additive, a heater support member made of an electrically insulating ceramic sintered body for supporting the heater element and an electric current supply means for supplying an electric current to the heater element. The composition of the mixture for the heater element is defined so as to satisfy the following formula: 0.035≦B/A&lt;0.35 where A is the amount of Si 3  N 4  expressed by mol percent of the total amount of the main ingredient of MoSi 2  and Si 3  N 4  and B is the amount of SiO 2  expressed by mol percent of the total amount of the main ingredient of MoSi 2  and Si 3  N 4 .

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a ceramic heater, especially a ceramicheater suitably employed as a glow plug in a diesel engine.

2. Description of the Prior Art

In diesel engines, glow plugs are employed for starting engines in anatmosphere of low temperatures and these glow plugs are required to beprovided with a heater having a rapidly heating property in order toimprove the startability of engines.

The present inventors have developed a ceramic heater comprising asupporting member made of electrically insulating ceramic sintered body,a heater element formed by bonding a ceramic sintered body of MoSi₂ andSi₃ N₄ to a top end of the supporting member, and lead wires embedded inthe supporting member and connected to the heater element (U.S.application Ser. Nos. 717,875 and 739,474).

This ceramic heater has oxidization resistance at high temperatures dueto the existence of MoSi₂ and thermal shock resistance due to theexistence of Si₃ N₄. Therefore, this ceramic heater can be employed in acombustion chamber of a diesel engine without being covered with anycovering member and accordingly, exhibits an excellent rapidly heatingproperty.

The temperature of glow plugs is controlled in accordance with thedriving condition of engines. In cold districts where the ambienttemperature is not more than -15° C., it takes time until the dieselengines become the state of good startability. So, the diesel enginesare earnestly desired to have good startability at low temperaturessimilar to gasoline engines, and accordingly, the heaters are desired togenerate heat of about 1300° C. to 1400° C. for the above purpose.

However, the ceramic heater made of MoSi₂ and Si₃ N₄ cannot maintaingood durability at a heating temperature of the heater as high as 1300°C. or more.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a ceramic heaterhaving excellent oxidization resistance, thermal shock resistance and agood rapidly heating property.

Another object of the present invention is to provide a ceramic heaterhaving excellent durability at temperatures as high as 1300° C. or more.

The ceramic heater of the present invention comprises a heater elementmade of a ceramic sintered body which generates heat upon receiving anelectric current, a heater support member made of an electricallyinsulating ceramic sintered body for supporting the heater element andan electric current supply means for supplying an electric current tothe heater element.

The heater element used in the ceramic heater of the present inventionis formed of a sintered body of a mixture including MoSi₂ and Si₃ N₄powder as a main ingredient and silica (SiO₂) powder as an additive.

The sintered body for the heater element is composed of MoSi₂, Si₃ N₄and Si₂ N₂ O converted from part of Si₃ N₄.

In the above described mixture, the composition thereof is defined so asto satisfy the following formula:

    0.035≦B/A<0.35

where A is the amount of Si₃ N₄ expressed by mol percent of the totalamount of the main ingredient of MoSi₂ and Si₃ N₄ and B is the amount ofSiO₂ expressed by mol percent of the total amount of the main ingredientof MoSi₂ and Si₃ N₄.

When the ratio B/A reaches 0.035, part of Si₃ N₄ is converted tosiliconoxinitride (Si₂ N₂ O) in the sintered body. Si₂ N₂ O is excellentin heat resistance, and especially oxidization resistance at hightemperatures and the existence of Si₂ N₂ O enables the long period usageof the heater element of which the heating temperature is not less than1300° C.

When the ratio B/A reaches 0.35, α-cristobalite is formed in thesintered body. This α-cristobalite is converted to β-cristobalite atabout 200° C. to generate cracks in the heater element due to distortionoccurring when α-cristobalite is converted to β-cristobalite. From thisreason, the existence of α-cristobalite is not preferable.

SiO₂ itself, aluminum silicate, silica glass, high silicate glass or thelike can be employed as the substance supplying SiO₂.

Preferably, the above described mixture is composed of 30 to 65 mol% ofMoSi₂ and 70 to 35 mol% of Si₃ N₄. The composition ratio of Si₃ N₄exceeds 70 mol%, the specific resistance of the heater element isundesirably increased though the thermal shock resistance thereof isimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a glow plug provided with a ceramic heateraccording to the present invention;

FIG. 2 is a view illustrating one producing step of the ceramic heater;

FIGS. 3, 4, 5, 6 and 7 are graphs showing the experimental results onthe ceramic heater according to the present invention; and

FIG. 8 is a view showing one example of conditions of one experiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 illustrates a glow plug provided with a ceramic heater accordingto the present invention.

The ceramic heater is composed of a rod-shaped supporting member 2formed of a ceramic sintered body, a heater element 1 having a U-shapedcross section, formed of a ceramic sintered body and bonded to an endsurface of the supporting member 2, and a pair of lead wires 3a and 3bembedded in the supporting member 2. A top end of each of the lead wires3a and 3b is connected to the heater element 1.

A metallic sleeve 4 is secured to the outer periphery of the supportingmember 2 and a metallic body 5 is secured to the outer periphery of themetallic sleeve 4.

A rear end of the lead wire 3a extends to a base end of the supportingmember 2 and is in contact with a metallic cap 6 fit in the base end ofthe supporting member 2.

The metallic cap 6 is electrically connected to an electric power source(not shown) through a nickel line 7. The glow plug having the abovedescribed structure is secured to an engine so as to be exposed to theinterior of a combustion chamber thereof by a screw 51 formed in themetallic body 5.

The supporting member 2 is formed of a sintered body of a mixture of Si₃N₄ and Al₂ O₃ (alumina).

The heater element 1 is formed of a sintered body of a mixture of MoSi₂,Si₃ N₄ and SiO₂. The composition ratio of each material of the abovemixture is adjusted so as to satisfy the following formula:0.035≦B/A<0.35 where A is the amount of Si₃ N₄ expressed by mol percentof the total amount of the main ingredient of MoSi₂ and Si₃ N₄ and B isthe amount of SiO₂ expressed by mol percent of the total amount of themain ingredient of MoSi₂ and Si₃ N₄.

In the obtained heater element, part of Si₃ N₄ is converted to Si₂ N₂ O.

FIG. 2 is a view explaining one producing step of the ceramic heateraccording to the present invention.

At first, MoSi₂ powder, Si₃ N₄ powder, SiO₂ powder and organic solventare mixed and a plurality of ceramic sheets 1' for the heater elementare formed from the obtained mixture by a doctor blade method.

Next, Si₃ N₄ powder, Al₂ O₃ powder and organic solvent are mixed and aplurality of ceramic sheets 2' for the supporting member are formed fromthe obtained mixture by a doctor blade method.

The obtained ceramic sheets 1' and 2' are piled on one another,respectively as shown in FIG. 2 and the lead wires 3a ad 3b are alsodisposed as shown in FIG. 2.

Then, the piled ceramic sheets 1' and 2' with the lead wires 3a and 3bare hot-pressed at a temperature of 1600° C. and under a pressure of 500Kg/cm² to obtain a ceramic heater shown in FIG. 1.

Hereinafter, the experimental results on the heater element of theceramic heater according to the present invention will be explained.

A mixture powder composed of MoSi₂ powder(average particle diameter: 0.9μm), Si₃ N₄ powder (average particel diameter: 35 μm) is mixed with SiO₂powder(average particle diameter: 1 μm) as an additive. Then, theobtained mixture is sintered to obtain a sintered body for a heaterelement.

The relation between the composition ratio of the materials and thespecific resistance of the obtained sintered body is shown in FIG. 3.

In FIG. 3, the composition ratio of Si₃ N₄ is shown by mol percent ofthe main ingredient of MoSi₂ and Si₃ N₄, and the composition ratio ofSiO₂ is shown by mol percent of the total amount of the main ingredientof MoSi₂ and Si₃ N₄.

As shown in FIG. 3, the specific resistance is increased with theincrease of the amount of SiO₂.

Next, the structure of the sintered bodies having different amounts ofSi₃ N₄ and SiO₂ is examined by a X ray analysis. The obtained result isshown in the following table. In this table, marks ○ mean existence ofsubstance and marks -- mean nonexistence of substance.

    ______________________________________                                         ##STR1##     0      0.035      0.14                                                                               0.35                                     ______________________________________                                        Si.sub.3 N.sub.4                                                                           O      O          O    O                                         MoSi.sub.2          O          O    O                                         Si.sub.2 N.sub.2 O                                                                         --     O          O    O                                         α-cristobalite                                                                       --     --         --   O                                         ______________________________________                                    

As shown in the above table, when the ratio B/A reaches 0.035, part ofSi₃ N₄ is converted to Si₂ N₂ O. Si₂ N₂ O is excellent in oxidizationresistance at high temperatures and accordingly serves to improve theheat resistance of the sintered body.

When the amount of additive SiO₂ is increased and the ratio B/A reaches0.35, α-cristobalite begins to be formed.

As described above, the existence of α-cristobalite is not preferablesince cracks are formed when the sintered bodies containingα-cristobalite are heated.

From the above result, the amount of SiO₂ for the heater element must bedefined so as to satisfy the formula 0.035≦B/A<0.35.

FIGS. 4 and 5 show the relation between the amount of SiO₂ added to themain ingredient of MoSi₂ -70 mol%Si₃ N₄ and MoSi₂ -35 mol%Si₃ N₄ and thedensity of the obtained sintered bodies.

In the former case, when the amount of SiO₂ is about 10 to 20 mol%, highdensity can be obtained and accordingly, heat resistance of the sinteredbody can be improved. When the amount of SiO₂ is 25 mol%, the ratio B/Ais larger than 0.35 and α-cristobalite is formed in the sintered body.

In the latter case, when the amount of SiO₂ is about 5 to 10 mol%, highdensity can be obtained and when the amount of SiO₂ is 13 mol%,α-cristobalite is formed in the obtained sintered body.

As is apparent from the experimental results shown in FIGS. 4 and 5, theaddition of SiO₂ in such an amount as satisfying the formula of0.035≦B/A<0.35 serves to increase the density of the obtained sinteredbody.

FIG. 6 shows the relation between the amount of additive SiO₂ and thethermal expansion coefficient of the obtained sintered bodies.

As is apparent from FIG. 6, in all of the sintered bodies of MoSi₂ -70mol%Si₃ N₄, MoSi₂ -50 mol%Si₃ N₄ and MoSi₂ -35 mol%Si₃ N₄, the thermalexpansion coefficient thereof scarcely increases by the addition of SiO₂in the amount within the range defined in the present invention.

FIG. 7 shows the result of examination on the variation of theresistance of a glow plug(resistance at a normal temperature is 0.18Ω)formed by sintering a mixture powder of MoSi₂ -70mol% Si₃ N₄ with anadditive powder of 25mol% of SiO₂ (B/A≈0.035) and securing the obtainedsintered body to a supporting member made of a sintered body of Si₃ N₄-50mol%Al₂ O₃.

In this experiment, the glow plug is subjected to a number of coolingand heating cycles as shown in FIG. 8 by intermittently applyingelectric current to the glow plug, and the variation of the resistanceof the heater elements of which the heating temperature is 1300° C. and1400° C., respectively, is examined.

As is shown in FIG. 7, the resistance of the heater element is changedonly within 10% when the amount of SiO₂ is 2.5 mol%.

In contrast, when the glow plug having a heater element made of asintered body of MoSi₂ -70mol%Si₃ N₄ without SiO₂ being added thereto issubjected to the same cooling and heating cycles, the resistance of theheating element is increased over 10% with the increase of the number ofcycles.

When the amount of SiO₂ is 25 mol%(B/A>0.35), the heater element isdamaged due to cracks formed therein at the moment of generating heat.

From the above experimental results, it has become clear that by addingnot less than 2.5 mol% of SiO₂ powder to a mixture powder of MoSi₂ -70mol%Si₃ N₄, Si₂ N₂ O having excellent oxidization resistance at hightemperatures is formed in the obtained heater element. Furthermore, inthis case, the density of the obtained heater element is increased asshown in FIGS. 4 and 5 to make the structure thereof compact. Thisresults in the heat resistance of the heater element is largelyimproved.

However, when the amount of additive SiO₂ exceeds 25 mol%,α-cristobalite is formed in the heater element. And when thisα-cristobalite is phase-transferred to α-cristobalite, distortion isgenerated to damage high temperature heater element.

As described above, the ceramic heater of the present invention isexcellent in its rapidly heating property and exhibits good durabilityat temperatures as high as 1300° C.

Accordingly, the ceramic heater of the present invention is suitable toa heater for a glow plug of a diesel engine.

What is claimed is:
 1. A ceramic heater comprising:a heater element madeof a ceramic sintered body which generates heat upon receiving anelectric current; a heater support member made of a electricallyinsulating ceramic sintered body, said heater element being affixed toone end of said heater support member; and an electric current supplymeans for supplying an electric current to said heater element; saidheater element being a sintered body of a mixture including MoSi₂ andSi₃ N₄ powder as a main ingredient and SiO₂ powder as an additive; saidsintered body being composed of MoSi₂, Si₃ N₄ and siliconoxinitrideconverted from part of Si₃ N₄ ; said mixture having a compositionexpressed by the formula: 0.035≦B/A<0.35 where A is the amount of Si₃ N₄powder expressec by mol percent of the total amount of said mainingredient and B is the amount of SiO₂ powder expressed by mol percentof the total amount of said main ingredient.
 2. A ceramic heateraccording to claim 1, wherein said main ingredient is composed of 30 to65 mol% of MoSi₂ powder and 70 to 35 mol% of Si₃ N₄ powder.
 3. A ceramicheater according to claim 1, wherein said SiO₂ powder is supplied from amaterial selected from the group consisting of silica, aluminumsilicate, silica glass and high silicate glass.
 4. A ceramic heateraccording to claim 1, wherein said electric current supply meanscomprises a pair of lead wires for connecting said heater element to anelectric power source, and said lead wires are embedded within saidheater support member.
 5. A ceramic heater according to claim 4, whereinsaid heater element is integrally sintered with said heater supportmember.